193 PATHOGENIC BACTERIA. 



point, so that this culture-medium, which is nearly trans- 

 parent, is almost as useful as gelatin. In addition to its 

 readiness to liquefy and solidify, it is sufficiently firm 

 to allow of the incubation-temperature — i. e. 37 ° C. — at 

 which gelatin is always liquid, and no better than bouillon. 



The preparation of this medium is generally described 

 in the text-books as one " requiring considerable patience 

 and much waste of filter-paper. " In reality, it is not dif- 

 ficult if a good heavy filter-paper be obtained and no 

 attempt be made to filter the solution until the agar-agar 

 is perfectly dissolved. It is prepared as follows : To 1000 

 c.cm. of bouillon made as described above, preferably of 

 meat instead of beef-extract, 10 to 15 grams of agar-agar 

 are added. The mixture is boiled vigorously for an 

 hour in an open pot over the direct gas flame or in the 

 double boiler with saturated calcium chloride solution 

 in the outside pot. After being cooled to about 6o° C. 

 and after titration an egg beaten up in water is added, 

 and the liquid is boiled again until the egg is entirely 

 coagulated. 



After the boiling the agar-agar is filtered, just as the 

 gelatin was, through a carefully-folded pharmaceutical 

 filter wet with boiling water. It may expedite matters 

 to pour in about one-half of the solution, keep the re- 

 mainder hot, and subsequently add it when necessary. 



The formerly much-employed hot-water and gas-jet 

 filters are unnecessary. If properly prepared, the whole 

 quantity will filter in from fifteen to thirty minutes. 



Ravenel l prepares his agar-agar by making two solu- 

 tions, one representing the meat-infusion, but twice the 

 usual strength, the other the agar-agar dissolved in one- 

 half the usual quantity of water. The agar-agar is dis- 

 solved by exposure to superheated steam in the autoclave, 

 after which the two solutions are poured together and 

 boiled until all of the albumins are precipitated. The 

 coagulation of the albumins of the meat-infusion serves 

 to clarify the agar-agar. 



x Journal of Applied Microscopy, June, 1898, vol. i., No. 6, p. 106. 



